In pulp mills, there is a digester, into which wood chips and a processing liquor are directed to be processed under conditions of high temperature and pressure. Generally, the digester comprises a quite large metallic container, with the walls of these containers having sufficient thickness and material strength to withstand these high pressures. Eventually, the liquor within the digester causes corrosion of the interior surface of the containing structure. This causes a weakening of the containing structure, and eventually, this requires the digester to be operated at lower temperatures and pressures. After a period of time, however, it becomes necessary either to replace the digester or to rehabilitate the digester so that it could be operated at the higher temperatures and pressures which enable the digesting process to be carried on more efficiently.
The usual method of accomplishing this is to shut-down the digester, empty the digester, and then reline the interior surface of the digester. This is commonly accomplished by welding metal beads along the entire inside surface of the digester to build up the thickness of the digester wall. This is an expensive, time consuming process, sometimes taking several weeks or more. Further, this can result in a prolonged shutdown of the mill, causing a very substantial loss of revenue.
Accordingly, there has for decades been a requirement for an improved means of rehabilitating such digesters.